Intrusion detection system employing quadrature sampling

ABSTRACT

An intrusion alarm system for the reliable detection of moving targets in a manner which discriminates against noise and other interfering phenomena. Signals returned from a zone under surveillance are processed by quadrature-phased detectors, producing a pair of Doppler signals. Zero crossings of one Doppler signal are used to produce pulses that are employed to sample the other Doppler signal, the sampled signal being stored and integrated to provide an integrated output signal. An alarm indication is produced upon exceedance of a predetermined threshold by the integrated output signal. In the presence of interfering phenomena, the sample pulses occur at random times causing a bipolar fluctuating integrated output signal which does not cross the predetermined threshold. In the presence of a true moving target, sample pulses of regular timed relation are provided to cause an integrated output signal of sufficient value to exceed the predetermined threshold. An alarm indication is thereby provided only in response to a true moving target.

United States Patent Galvin et al.' 4

[ Sept. 18, 1973 [75] Inventors: Aaron A. Galvin, Lexington; L.

Dennis Shapiro, Lincoln, both of Mass.

[73] Assignee: Aerospace Research, Inc., Boston,

Mass.

[22] Filed: Feb. 10, 1972 211 Appl. No.: 225,049

[52] Cl 340/258 A, 340/276, 343/7.7 [51] Int. Cl. G08b 13/24 [58] Fieldof Search 340/258 A, 276; 343/7.7

[56] References Cited UNITED STATES PATENTS 3,331,065 7/1967 McDonald340/258 A 3,665,443 5/1972 Galvin 340/258 A PrimaryExamI'nerJohn W.Caldwell Assistant Examiner-Glen R. Swann, Ill Attorney-JosephWeingarten et al.

J10 TRANSMITTING RECEIVING TRANSDUCER TRANSDUCER [5 7] ABSTRACT Anintrusion alarm system for the reliable detection of moving targets in amanner which discriminates against noise and other interferingphenomena. Signals returned from a zone under surveillance are processedby quadrature-phased detectors, producing a pair of Doppler signals.Zero crossings of one Doppler signal are used to produce pulses that areemployed to sample the other Doppler signal, the sampled signal beingstored and integrated to provide an integrated output signal. An alarmindication is produced upon exceedance of a predetermined threshold bythe integrated output signal. In the presence of interfering phenomena,the sample pulses occur at random times causing a bipolar fluctuatingintegrated output signal which does not cross the predeterminedthreshold. In the presence of a true moving target, sample pulses ofregular timed relation are provided to cause an. integrated outputsignal of sufficient value to exceed the predetermined threshold. Analarm indication is thereby provided only in response to a true movingtarget.

l3 Claims, 16 Drawing Figures I6 ouAoRATuRE REE MIxERs DOPPL R BANDPAESSB VERY-HIGH-GAIN C FILTER a LIMITING DIFFERENTIATOR AMPLIFIER AMPLIFIERI as L ow PASS BIAS POSITIVE FILTER PULSE AMPLIFIER E 26 DOPPLER IBANDPASS A SYMMETRICAL sAMPLEaHoLD F FILTERBI 'LIMITER CIRCUITINTEGRATOR AMPLIFIER (2o 22 24 I 28 BIPOLAR THRESHOLD CIRCUIT l M ALARINDICATOR Patented Sept. 18, 1973 4 Sheets-Sheet 1 Patented Sept. 18,1973 4 Sheets-Sheet 2.

FIG. 2A

FIGZB FIG.2C

' FIGZD FIG. 2E

FIGZF +THRESHOLD THRESHOLD FIG.2G

Patented Sept. 18, 1973 4 Sheets-Sheet FIG.3C

Urmx

if W

FIG.3D

FIG.3E

I THRESHOLD FIG.3G

- THRESHOLD Patented Sept. 18, 1973 4 Sheets-Sheet 4 FROM DIFFERENTIATORy:

POSITIVE PULSE AMPLIFIER FROM LIMITER Q SAMPLE V 8: HOLD CIRCUITNEGATIVE PULSE AMPLIFIER INTEGRATOR SAMPLE 8x HOLD CIRCUIT INVERTER FIG.4

TO THRESHOLD CIRCUIT 2 INTRUSION DETECTION SYSTEM EMPLOYING QUADRATURESAMPLING FIELD OF THE INVENTION BACKGROUND OF THE INVENTION Inelectromagnetic intrusion alarm systems operative typically atultrasonic or radar frequencies, a transmitted signal is directed into azone under surveillance and a signal returned from a moving targetwithin the zone is mixed with a portion of the transmitted signal toproduce a beat frequency representative of the Doppler frequency of themoving target and which is sensed and employed to energize a suitablealarm. The efficiency and commercial utility of such intrusion alarmsystems depend in large measure on the system ability todiscriminatebetween true moving targets and interfering phenomena caused byconditions present in the surveillance zone. Interfering phenomena in azone under surveillance are typically the result of movement of doorsand windows, vibrating walls, and moving air currents, which can causespurious alarm indications.

Techniques proposed heretofore for reducing the tendency of false alarmshave, in general, employed gain orthreshold compensation based upon ameasurement of background noise or clutter averaged over a period oftime. This type of gain or threshold variation is effective only if theinterfering phenomena has a gradual onset. In practice, however,interfering phenomena often do notv exhibit a gradual onset, but,rather, are abrupt such as caused by the starting and stopping of airconditioners, heaters and objects which spuriously move and vibrate.Gain or threshold compensation techniques are therefore not generallyvery effective against those spurious conditions often encountered in anactual working environment. In addition, variation of the gain of asystem also causes variable detection sensitivity which also results invariable target-detection range.

Another prior art technique for discriminating true moving targets frominterfering phenomena has employed selective filtering to eliminate amajor portion of Doppler noise energy caused by the interferingphenomena, while retaining sensitivity against a target moving at arelatively high radial velocity. Such systems sacrifice detection oftargets moving at low radial velocities since, for such low velocities,the Doppler frequencies attributable to interfering phenomena aregenerally the same as those attributable to a true moving target.

A particularly effective system for discriminating moving targets frominterfering phenomena is disclosed and claimed in the copendingapplication of Aaron A. Galvin, entitled Ultrasonic Intrusion Alarm,Ser. No. 69,306, filed Sept. 3, I970, now 0.8. Pat. 3,665,443 andassigned to the assignee of the present invention, and in which certainspectral characteristics of moving targets and interfering phenomena areemployed to achieve improveddiscrimination of moving targets in thepresence of such phenomena. According to the system of the copendingapplication, a pair of output signals is provided in response to areceived signal the spectrum of which is balanced with respect'to thetransmitted carrier, while producing only one or the other of the pairof output signals in response to a received signal the spectrum of whichis unbalanced with respect to the carrier. The spectrum caused by a truemoving target is assymmetrical with respect to the carrier frequencycausing one first or second output signal, and the alarm circuitry isconfigured to provide an alarm indication only in response to the firstor the second output signal but not when both output signals are presentin comparable proportion. The present invention provides a system fordetecting moving targets from interfering phenomena with theeffectiveness of the system of the copending application but utilizing adifferent technique of discrimination which is readily implemented byrelatively simple circuitry.

SUMMARY OF THE INVENTION In accordance with the present invention, anintrusion alarm system is provided in which quadrature phased signalsare digitally decoded to provide an output indication of moving targetpresence and which is capable of a high degree of discrimination betweentrue moving targets and interfering phenomena. A transmitted signal isdirected into an area under surveillance and signals returned fromobjects and targets within the zone are received and processed in a pairof quadrature mixers to provide a pair of Doppler signals usually ataudio frequencies. One of these signals is processed to derive samplepulses at its positive zero axis crossings. For a given target Dopplerreturn, these pulses will occur at or near the peaks of the otherDoppler signal. A representation of the sampled peak signal is storedand integrated to provide an integrated output signal which is comparedwith a reference bipolar threshold, and upon exceedance of the thresholdby the integrated output signal, and alarm indication of target presenceis produced. v V

In the presence of interfering phenomena, the sample pulses occur atrandom times causing a bipolar fluctuating integrated output signalwhich does not cross the predetermined threshold. In the presence of atrue moving target, sample pulses of regular timed relation are providedto cause an integrated output signal of sufficient value to exceed thepredetermined threshold. An alarm indication is thereby provided only inresponse to a true moving target.

In a typical embodiment of the invention, a first one of the Dopplersignals is processed by a symmetrical limiter and applied to the signalinput of a sample and hold circuit. The other of the Doppler signals isprocessed by a very-high-gain limiting amplifier which provides arectangularly shaped output in response to received input signals. Therectangularly shaped output of the very-high-gain limiting amplifier isdifferentiated and the positive-going portion of the waveform isamplified to provide sample pulses to the trigger input of the sampleand hold circuit. The sample pulses occur at the positive zero axiscrossings of the very-high-gain limiting amplifier's output signal. Theoutput of the sample and hold circuit is applied to a bipolar integratorwhich will integrate both positive and negative signals. The integratoroutput is applied to a bipolar threshold circuit the output of whichdrives an alarm indicator or other utilization apparatus.

Under conditions of interfering phenomena, such as broadband noise orclutter, the polarity of signals stored by the sample and hold circuitat the time of the sample pulse will be random in nature and, thus, theoutput signal from the sample and hold circuit will tend to vary in agenerally symmetrical manner both above and below a reference level,generally taken to be system ground. The integrator under noiseconditions will not charge sufficiently in either a positive or negativedirection to exceed a predetermined threshold for alarm actuation. Inthe presence of a moving target, however, the sample pulse will occur ator near the positive or negative peak of each cycle of the quadraturechannel signal, one polarity occurring for incoming targets and theother for outgoing targets. As a result, a unidirectional target willcause either a positive or negative charging of the integrator, theintegrator output signal, upon exceedance of the predeterminedthreshold, causing an alarm indication of target presence.

DESCRIPTION OF THE DRAWINGS The invention will be more fully understoodfrom the following detailed description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram representation of an intrusion alarm systemaccording to the invention;

FIGS. 2A 26 are waveforms useful in illustrating operation of theinvention in the presence of a moving target;

FIGS. 3A 30 are waveforms useful in illustrating operation of theinvention in the presence of interfering phenomena; and

FIG. 4 is a block diagram representation of an'alternative embodiment ofthe system of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION I An intrusion alarm systemconstructed and operative according to the invention is illustrated inFIG. 1 and includes a transmitting transducer energized by a signal froman oscillator 12 and operative to direct energy in a predeterminedpattern within a zone under surveillance. Energy returned from thesurveillance zone is received by a receiving transducer 14, the outputof which is applied to one input of a pair of quadrature mixers 16, theother input of which is a reference signal derived from oscillator 12.Quadrature mixers 16 provide a pair of Doppler output signals which arerespectively applied to first and second Doppler bandpass filters and amplifiers l8 and 20. For a carrier frequency of 26 KHz, Doppler signalsare typically provided in the range of 1000 Hz. The output of bandpassfilter and amplifier 20 is coupled to a symmetrical limiter 22 theoutput of which is applied to the signal input of a sample and holdcircuit 24. The limiter 22 provides linear operation for applied signalswithin a predetermined amplitude range and limits signals which exceedthe boundaries of this range. The output of sample and hold circuit 24is coupled to an integrator circuit 26 which is of bipolar form tooperate on either positive or negative signals and to produce a positiveor negative going integrated output signal accordingly. The outputsignal of integrator 26 is applied to a bipolar threshold circuit28which provides an output signal to actuate an alarm indicator 30, orother utilization apparatus.

The output of bandpass filter and amplifier 13 is coupled to avery-high-gain limiting amplifier 32 which is operative to provide arectangular shaped output signal in response to effectively all inputsignals applied thereto. The output of limiting amplifier 32 is coupledto a differentiator 34, the output of which, in turn, is coupled to apositive pulse amplifier 36, the output of which provides sample pulsesto sample and hold circuit 24. The pulse amplifier 36 is preferably ofthe type which includes an internal threshold such that only the signalsapplied thereto above this threshold are amplified to produce narrowpulses for effective sampling of the signal stored in sample and holdcircuit 24. In the illustrated embodiment, pulse amplifier 36 operateson the positive polarity pulses provided by differentiator 34; the pulseamplifier can however just as well operate on the negative polaritydifferentiator pulses if desired in a particular circuit implementation.

Under conditions when a moving target produces low signal amplitudes atlow Doppler frequencies, relatively high levels of hum or noisesuperimposed on the Doppler signal can cause spurious zero-axiscrossings from which erroneous sample pulses are derived. Such spurioussample pulses can occur at times when the signal being sampled is ofopposite polarity to that intended to be sampled, with the result thatsignificant error in the integrated output signal can arise by impropersampling. To substantially minimize false sampling caused by such noiseconditions, a low pass filter 37 is provided for receiving the outputsignal from very-highgain limiting amplifier 32 and for producing a biassignal to pulse amplifier 36. Under conditions of low Doppler frequencysignals, a time varying bias signal is applied by filter 37 to pulseamplifier 36 to prevent production of sample pulses by the pulseamplifier at times at which sample pulses should not occur.

The output waveform of the low pass filter 37 is of generallyrtriangularform having peaks occurring at the transitions of the rectangularwaveform provided by limiting amplifier 32.

When the bias level provided by filter 37 is above a predeterminedreference value, in the vicinity of the positive peaks of the low passfilter output signal, the pulse amplifier will not produce samplepulses. As a result, most of the improperly occurring sample pulsescaused by noise will be inhibited by action of the low pass filter. Athigher Doppler frequencies, above the cutoff point of filter 37, whenbias adjustment is not needed, no pulse inhibition is provided. By wayof example, for a carrier frequency of 26 KHz, Doppler signals of Hz orless produced by a target moving at about two feet per second or less,can experience improper sampling due to noise components present, andsuch false sampling is substantially prevented by the bias techniquedescribed.

The operation of the system of FIG. 1 in the presence of a moving targetwill be discussed in conjunction with the waveforms of FIGS. 2A through26. The letter designations'of the waveform diagrams correspond to thepoints of FIG. 1 similarly lettered. FIG. 2A depicts the Dopplerfiltered output signal of bandpass filter and amplifier 20, while FIG.2B depicts the similar signal from bandpass filter and amplifier 18which is in phase quadrature with the signal from circuit 20. Theillustrated waveform of- FIG. 2A in solid line is representative of thatwhich would occur for a target moving toward the receiver, the dottedwaveform representing a target moving away from the receiver and whichis, accordingly, of phase difference to the waveform produced by anincoming target.

After processing of the waveform of FIG. 2B by veryhigh-gain limitingamplifier 32, a rectangular wave signal shown in FIG. 2C is produced atthe limiter output. The rectangular wave signal is differentiated bydifferentiator 34 to produce the pulses shown in FIG. 2D which are ofalternate positive and negative polarity in accordance with the positiveor negative going sense of the rectangular wave being differentiated.The differentiator output pulses are applied to pulse amplifier 36responsive to signals of positive polarity and which produces outputpulses as shown in FIG. 2E. As discussed, the pulse amplifier 36includes an internal threshold such that only the positive peaks of theinput signals applied thereto are amplified to provide the narrow pulsesI of FIG. 2E occurring in time at the zero crossings of the rectangularwave output from limiting amplifier 32.

The signal from bandpass filter and I amplifier shown in FIG. 2A isapplied to a symmetrical limiter 22 which provides limiting action onlywhen the input signal exceeds preset upper and lower reference levels toprevent signal excursions which can cause overloading of the sample andhold circuit, which, in turn, can produce asymmetries resulting involtage offsets and/or sampled signal levels which drive the integratortoo rapidly, both of which could cause false alarms. The sample and holdcircuit 24 retains the potential of the signal from limiter 22 at thetimes specified by the sample pulses from pulse amplifier 36, the outputsignal of the sample and hold .circuit being depicted in FIG. 2F. Thesolid waveform of FIG. 2F is produced in response to a target movingwith one sense, while the dotted waveform is produced for a targetmoving with opposite sense. As depicted in FIG. 2F, the output signalfrom the sample and hold circuit decreases slightly due to dischargebetween sample pulses of the capacitor within the sample and holdcircuit. Each subsequent sampling pulse provided by pulse amplifier 35causes recharging of the circuit capacitor to the sampled level of theinput signal applied from limiter 22.

The integrator 26 provides, in response to the output signal of sampleand hold circuit 24, either the solid line or dotted line waveform shownin FIG. 2G,'de-

pending on the sense of the moving target either toward signal ispositive going and upon crossing of the positive threshold, causes alarmindication. If the target isoutgoing rather than incoming, the outputsignal of integrator 26 is negative going and upon crossing of thenegative threshold, causes alarm actuation. The threshold levels aredetermined in accordance with the amplitude and duration of expectedtarget signals.

The system described above is thus operative in the presence of targetsignals to sample the target signal at the positive or negative peak ofeach cycle depending upon the relative sense of the moving target, thesampled signal providing a unidirectional charging of an integrator toprovide an output signal which upon exceedance of a predeterminedthreshold level causes alarm actuation. The system will produce no alarmindication in the presence of noise or other interfering phenomena andis substantially immune to false alarms in the presence of such noise orspurious signal conditions. The alarm indication may be an audible orvisual alarm, or a signal indication of target detection fortransmission to a remote receiving site such as a guard or policestation.

Operation of the system of FIG. I in the presence of clutter and noiseis discussed in conjunction with the waveforms of FIGS. 3A through 3G.Under such noise conditions, the Doppler signals from quadrature mixersI6, after bandpass filtering and amplification by respective circuits 20and I4, appear as illustrated in FIGS. 3A and 3B which are seen to besignals having random amplitude and sense.

After high gain limiting by limiting amplifier 32, the signal frombandpass filter and amplifier I3 is as shown in FIG. 3C and which is arectangularly shaped signal provided in response to the zero crossingsof the waveform of FIG. 3B. The differentiator output of differentiator34 is as depicted in FIG. 3D, the positive differentiator output pulsesbeing, after amplification by positive pulse amplifier 36, as depictedin FIG. 3E and which occur at random times depending upon the time ofthe zero crossings of thewaveform of FIG. 3B. The waveform of FIG. 3A isthus sampled at random times and since the waveform of FIG. 3A has azero mean value, the resultant output signal from sample and holdcircuit 24, which is shown in FIG. 3F, also has zero mean value, theresultant output signal from sample and hold circuit 24, which is shownin FIG. 3F, also has zero mean value. The integrator output signal shownin FIG. 3G and which is provided in response to the output signal fromsample and hold circuit 24 will not exceed the predetermined thresholdprovided by threshold circuit 28 but rather will fluctuate above andbelow a zero reference line. No alarm indication is provided since thepreset thresholds are not exceeded by the integrator output signal underthe illustrated conditions of clutter and noise.

It will be appreciated that the embodiment of the invention describedabove is of half-wave configuration being operative in response toeither positive or negative Doppler signals to produce one sample percycle. A full-waveembodiment of the invention is depictedin FIG. 4,wherein two samples per Doppler cycle are provided and which can resultin a lower fluctuation level at the output of the integrator in thepresence of noise conditions.

Referring to FIG. 4, the signal from limiter 22 is applied to sample andhold circuit 24 the output of which is applied to one input ofintegrator 26 as described above in connection with the-system of FIG.I. Sample pulses for sample and hold circuit 24 are provided by positivepulse amplifier 36 which receives the differentiated signals fromdifferentiator 34. The signals from limiter 22 are also applied to aninverter circuit 40 the output of which is applied to a second sampleand hold circuit 42. The output of circuit 42 is applied to an input ofintegrator 26. Sample pulses for sample and hold circuit 42 areprovidedby a negative pulse amplifier 44 which also receives input signals fromdifferentiator 34. The pulse amplifier 36 is responsive to positivesignals from the differentiator, while pulse amplifier 44 is responsiveto negative signals therefrom. Integrator 26 is as noted above a bipolarintegratoroperative to provide an output signal of either positive ornegative sense in accordance with the polarity of input signals appliedthereto. The output of integrator 26 is applied to threshold circuit 28,as in the system of FIG. 1. Although not illustrated in FIG. 4, the biasadjustment of pulse amplifiers 36 and 44 by low pass filtering of thesignal from limiter 32 can be provided as in the embodiment of FIG. 1 tominimize false sampling caused by noise under low Doppler frequencysignal conditions.

In operation, the pulse amplifier 36 will produce sample pulses inresponse to the positive peaks of the differentiator output signal,while the pulse amplifier 44 will produce sample pulses in response tothe negative peaks of the differentiator output signal. Two samplepulses per Doppler cycle are thereby provided which will cause, in thepresence of noise, a more random and thus a lower fluctuation level inthe integrator output signal to further reduce the chance forinadvertent exceedance of the system thresholds to further minimizefalse alarms. In the presence of a true moving target, the full wavesampling of received quadrature signals permits the output signal fromthe sample and hold circuits to more nearly maintain its maximum outputvalue, as there is a shorter interval between sample pulses.

It will be appreciated that the invention can be embodied in a varietyof configurations and can be implemented in various ways to suitparticular operating requirements. Accordingly, it is not intended tolimit the invention by what has been particularly shown and described,except as indicated in the appended claims.

What is claimed is:

1. An intrusion alarm' system for detecting the presence of a movingtarget in the presence of interfering phenomena, said system comprising:

means for transmitting signals into a zone under sur veillance;

means for receiving signals returned from said surveillance zone;

means operative in response to said received signals to provide firstand second signals which include Doppler signals in. the presence andrepresentative of a moving target; means operative in response to saidfirst signal to produce sample pulses which occur in regular timedrelation in the presence of a moving target and at random times in thepresence of interfering phenomena;

means for storing a representation of the amplitude of said secondsignal at times specified by said sample pulses;

integrator means for integrating said representation of the amplitude ofsaid second signal and providing an output signal representativethereof, said output signal being of unidirectional increasing magnitudein the presence of a moving target and of randomly varying magnitude inthe presence of interfering phenomena;

threshold means for providing at least one threshold level and an outputsignal only upon exceedance of said at least one threshold level by saidintegrator output signal; and alarm means for providing an alarmindication in response to the output signal of said threshold means.

2. An intrusion alarm system according to claim 1 wherein said meansoperative in response to said first signal includes:

a limiting amplifier operative to provide a rectangular shaped outputsignal in response to said first signal applied thereto; differentiatormeans operative in response to said 5 rectangular shaped output signalto provide pulses each occurring during a zero crossing of saidrectangular shaped output signal, said pulses being of alternatepositive and negative sense; and a pulse amplifier for amplifying saiddifferentiator pulses of one sense to provide said sample pulses. 3. Anintrusionalarm system according to claim 1 wherein said means operativein response to said first signal includes:

a limiting amplifier operative to provide a rectangular shaped outputsignal in response to said first signal applied thereto; differentiatormeans operative in response to said output signal to provide pulses eachoccurring dur-' ing a zero crossing of said rectangular shaped out- 2o'put signal, said pulses being of alternate positive and negative sense;and pulse amplifier means for amplifying said differentiator pulses ofboth positive and negative sense to provide said sample pulses.-

4. An intrusion alarm system according to claim 1 wherein said means forproducing sample pulses includes means operative in response to receivedDoppler signals of predetermined low frequency and having noisecomponents superimposed thereon to inhibit production of sample pulsesin the presence of spurious signals caused by said noise components.

5. An intrusion alarm system according to claim 1 wherein said means forstoring a representation of the amplitude of said second signalincludes:

limiter means receiving said second signal and providing an outputsignal representation of said second signal limited to prevent signalexcursions above predetermined reference levels; and sample and holdmeans receiving said output signal representation from said limitermeans and operative tostore the representation of the amplitude of saidsecond signal at times specified by said sample pulses. v

system according'to claim 2 in- 6. An intrusion alarm cluding: 7

low pass'filter meansfor receiving said rectangular shaped output signalfrom said limiting amplifier and for providing in response thereto abias signal for said pulse amplifier to inhibit said sample pulses inthe presence of spurious zero crossings of said rectangular shapedoutput signal caused by low Doppler frequency signals having noisecomponents superimposed thereon.

7. An intrusion alarm system according to claim 1 wherein said meansoperative in response to said first signal includes! a limitingamplifier operative in response to said first signal to provide arectangular shaped outputsignal;

a differentiator circuit operative in response to said rectangularshaped output signal to provide pulses each occurring during a zerocrossing of said rectangular shaped output signal;

pulse amplifier means for amplifying said differentiator pulses of atleast one polarity to provide said sample pulses which occursubstantially at peaks of said second signal in the presence of a movingtarget;

low pass filter means operative in response to said rectangular shapedoutput signal to provide a control signal to said pulse amplifier meansto inhibit said sample pulses in the presence of spurious zero crossingsof said rectangular shaped output signal caused by noise superimposed onlow Doppler frequency signals;

and wherein said means for storing a representation of the amplitude ofsaid second signal includes:

a limiter circuit for receiving said second signal and providing inresponse thereto an output signal representation of said second signallimited to prevent signal excursions above predetermined referencelevels; and

a sample and hold circuit for receiving said output signalrepresentation from said limiter circuit and operative to store therepresentation of the amplitude of said second signal at times specifiedby said sample pulses.

8. An intrusion alarm system according to claim 2 wherein said means forstoring a representation of the amplitude of said second signalincludes:

limiter means receiving said second signal and providing an outputsignal representation of said second signal limited to prevent signalexcursions above predetermined reference levels; and

sample and hold means receiving said output signal representation fromsaid limiter means and operative to store the representation of theamplitude of said second signal at times specified by said samplepulses.

9. An intrusion alarm system according to claim. 7 wherein saidthreshold means is a bipolar threshold circuit having a positivethreshold for moving targets of one sense and a negative threshold formoving targets of opposite sense; and

wherein said integrator means is operative to provide a unidirectionaloutput signal of either positive or negative sense in accordance withthe sense of target motion.

10. An intrusion alarm system for detecting the presence of a movingtarget in the presence of interfering phenomena, said system comprising:

means for transmitting signals into a zone under surveillance;

means for receiving signals returned from said surveillance zone;

means operative'in response to said received signals toprovide first andsecond signals which include Doppler signals in the presence andrepresentative of a moving target;

means operative in response to said first signal to produce samplepulses which occur in regular timed relation in the presence of a movingtarget andat random times in the presence of interfering phenomena, andincluding:

a limiting amplifier operative to provide a rectangular shaped outputsignal in response to said first sig nal applied thereto; 1

differentiator means operative in response to said rectangular shapedoutput signal to provide pulses each occurring during a zero crossing ofsaid rectangular shaped output signal, said pulses being of alternatepositive and negative sense; and

ll lil first and second pulse amplifiers respectively opera tive toamplify said differentiator pulses of positive and negative sense toprovide first and second trains of sample pulses;

limiter means receiving said second signal and providing an outputsignal representation of said second signal limited to prevent signalexcursions above predetermined reference levels;

first sample and hold means receiving said output signal representationfrom said limiter means and operative to store the representation of theamplitude of said second signal at times specified by said first trainof sample pulses; means for inverting said output signal representationfrom said limiter means; and second sample and hold means receiving saidinverted output signal representation and operative to store therepresentation of the amplitude of said second signal at times specifiedby said second train of sample pulses; integrator means for integratingsaid representation of the amplitude of said second signal and providingan output signal representative thereof, said output signal being ofunidirectional increasing magnitude in the presence of a moving targetand of randomly varying magnitude in the presence of interferingphenomena; threshold means for providing at least one threshold leveland an output signal only upon exceedance of said at least one thresholdlevel by said integrator output signal; and alarm means for providing analarm indication in response to the output signal of said thresholdmeans. ill. An intrusion alarm system according to claim 10 wherein saidthreshold means is a bipolar threshold circuit having a positivethreshold for moving targets of one sense and a negative threshold formoving targets of opposite sense; and

wherein said integrator means is operative to provide a unidirectionaloutput signal of either positive or negative sense in accordance withthe sense of target motion. 12. An intrusion alarm system according toclaim lltll including:

low pass filter means for receiving said rectangular shaped outputsignal from said limiting amplifier and for providing in responsethereto a bias signal for said pulse amplifiers to inhibit said samplepulses in the presence of spurious zero crossings of said rectangularshaped output signal caused by low Doppler frequency signals havingnoise components superimposed thereon. l3. An intrusion alarm system fordetecting the presence of a moving target in the presence of interferingphenomena, said system comprising:

means for transmitting a signal into a zone under surveillance; meansfor receiving signals returned from said surveillance zone; quadraturemixer means operative in response to said received signals and to areference signal derived from said transmitted signals to provide firstand second signals which include Doppler signals in the presence of amoving target; means operative in response to said first signal toproduce sample pulses which occur in regular timed relation in thepresence of a moving target and at Ell random times in the presence ofinterfering phenomena, said sample pulse producing means including:

a limiting amplifier operative to provide a rectangular shaped outputsignal in response to said first signal;

a differentiator circuit operative in response to said rectangularshaped output signal to provide pulses each occurring during a zerocrossing of said rectangular shaped output signal;

pulse amplifier means operative to amplify said differentiator pulses ofat least one polarity to provide said sample pulses; and

low pass filter means for receiving said rectangular shaped outputsignal from said limiting amplifier and for providing in responsethereto a control signal for said pulse amplifier means to inhibit saidsample pulses in the presence of spurious zero axis crossings of saidrectangular shaped output signal caused by low Doppler frequency signalshavingnoise components superimposed thereon;

a limiter circuit operative in response to said second signal to providean output signal representation of said second signal limited to preventsignal excursions above predetermined reference levels;

sample and hold means receiving said output signal representation fromsaid limiter circuit and operative to store the representation of theamplitude of said second signal at times specified by said samplepulses; I

an integrator circuit for providing an integrated output signal inresponse to said stored representation of the amplitude of said secondsignal, said integrated output signal being of unidirectional increasingmagnitude in the presence of a moving target and of randomly varyingmagnitude in the presence of interfering phenomena; and

a threshold circuit having at least one threshold level which is outsidethe range of said randomly varying integrated output signal, andoperative to produce an alarm signal only upon exceedance of said atleast one threshold level by said integrated output signal.

1. An intrusion alarm system for detecting the presence of a movingtarget in the presence of interfering phenomena, said system comprising:means for transmitting signals into a zone under surveillance; means forreceiving signals returned from said surveillance zone; means operativein response to said received signals to provide first and second signalswhich include Doppler signals in the presence and representative of amoving target; means operative in response to said first signal toproduce sample pulses which occur in regular timed relation in thepresence of a moving target and at random times in the presence ofinterfering phenomena; means for storing a representation of theamplitude of said second signal at times specified by said samplepulses; integrator means for integrating said representation of theamplitude of said second signal and providing an output signalrepresentative thereof, said output signal being of unidirectionalincreasing magnitude in the presence of a moving target and of randomlyvarying magnitude in the presence of interfering phenomena; thresholdmeans for providing at least one threshold level and an output signalonly upon exceedance of said at least one threshold level by saidintegrator output signal; and alarm means for providing an alarmindication in response to the output signal of said threshold means. 2.An intrusion alarm system according to claim 1 wherein said meansoperative in response to said first signal includes: a limitingamplifier operative to provide a rectangular shaped output signal inresponse to said first signal applied thereto; differentiator meansoperative in response to said rectangular shaped output signal toprovide pulses each occurring during a zero crossing of said rectangularshaped output signal, said pulses being of alternate positive andnegative sense; and a pulse amplifier for amplifying said differentiatorpulses of one sense to provide said sample pulses.
 3. An intrusion alarmsystem according to claim 1 wherein said means operative in response tosaid first signal includes: a limiting amplifier operative to provide arectangular shaped output signal in response to said first signalapplied thereto; differentiator means operative in response to saidoutput signal to provide pulses each occurring during a zero crossing ofsaid rectangular shaped output signal, said pulses being of alternatepositive and negative sense; and pulse amplifier means for amplifyingsaid differentiator pulses of both positive and negative sense toprovide said sample pulses.
 4. An intrusion alarm system according toclaim 1 wherein said means for producing sample pulses includes meansoperative in response to received Doppler signals of predetermined lowfrequency and having noise components superimposed thereon to inhibitproduction of sample pulses in the presence of spurious signals causedby said noise components.
 5. An intrusion alarm system according toclaim 1 wherein said means for storing a representation of the amplitudeof said second signal includes: limiter means receiving said secondsignal and providing an output signal representation of said secondsignal limited to prevent signal excursions above predeterminedreference levels; and sample and hold means receiving said output signalrepresentation from said limiter means and operative to store therepresentation of the amplitude of said second signal at times specifiedby said sample pulses.
 6. An intrusion alarm system according to claim 2including: low pass filter means for receiving said rectangular shapedoutput signal from said limiting amplifier and for providing in responsethereto a bias signal for said pulse amplifier to inhibit said samplepulses in the presence of spurious zero crossings of said rectangularshaped output signal caused by loW Doppler frequency signals havingnoise components superimposed thereon.
 7. An intrusion alarm systemaccording to claim 1 wherein said means operative in response to saidfirst signal includes: a limiting amplifier operative in response tosaid first signal to provide a rectangular shaped output signal; adifferentiator circuit operative in response to said rectangular shapedoutput signal to provide pulses each occurring during a zero crossing ofsaid rectangular shaped output signal; pulse amplifier means foramplifying said differentiator pulses of at least one polarity toprovide said sample pulses which occur substantially at peaks of saidsecond signal in the presence of a moving target; low pass filter meansoperative in response to said rectangular shaped output signal toprovide a control signal to said pulse amplifier means to inhibit saidsample pulses in the presence of spurious zero crossings of saidrectangular shaped output signal caused by noise superimposed on lowDoppler frequency signals; and wherein said means for storing arepresentation of the amplitude of said second signal includes: alimiter circuit for receiving said second signal and providing inresponse thereto an output signal representation of said second signallimited to prevent signal excursions above predetermined referencelevels; and a sample and hold circuit for receiving said output signalrepresentation from said limiter circuit and operative to store therepresentation of the amplitude of said second signal at times specifiedby said sample pulses.
 8. An intrusion alarm system according to claim 2wherein said means for storing a representation of the amplitude of saidsecond signal includes: limiter means receiving said second signal andproviding an output signal representation of said second signal limitedto prevent signal excursions above predetermined reference levels; andsample and hold means receiving said output signal representation fromsaid limiter means and operative to store the representation of theamplitude of said second signal at times specified by said samplepulses.
 9. An intrusion alarm system according to claim 7 wherein saidthreshold means is a bipolar threshold circuit having a positivethreshold for moving targets of one sense and a negative threshold formoving targets of opposite sense; and wherein said integrator means isoperative to provide a unidirectional output signal of either positiveor negative sense in accordance with the sense of target motion.
 10. Anintrusion alarm system for detecting the presence of a moving target inthe presence of interfering phenomena, said system comprising: means fortransmitting signals into a zone under surveillance; means for receivingsignals returned from said surveillance zone; means operative inresponse to said received signals to provide first and second signalswhich include Doppler signals in the presence and representative of amoving target; means operative in response to said first signal toproduce sample pulses which occur in regular timed relation in thepresence of a moving target and at random times in the presence ofinterfering phenomena, and including: a limiting amplifier operative toprovide a rectangular shaped output signal in response to said firstsignal applied thereto; differentiator means operative in response tosaid rectangular shaped output signal to provide pulses each occurringduring a zero crossing of said rectangular shaped output signal, saidpulses being of alternate positive and negative sense; and first andsecond pulse amplifiers respectively operative to amplify saiddifferentiator pulses of positive and negative sense to provide firstand second trains of sample pulses; limiter means receiving said secondsignal and providing an output signal representation of said secondsignal limited to prevent signal excursions above predeterminedreference levelS; first sample and hold means receiving said outputsignal representation from said limiter means and operative to store therepresentation of the amplitude of said second signal at times specifiedby said first train of sample pulses; means for inverting said outputsignal representation from said limiter means; and second sample andhold means receiving said inverted output signal representation andoperative to store the representation of the amplitude of said secondsignal at times specified by said second train of sample pulses;integrator means for integrating said representation of the amplitude ofsaid second signal and providing an output signal representativethereof, said output signal being of unidirectional increasing magnitudein the presence of a moving target and of randomly varying magnitude inthe presence of interfering phenomena; threshold means for providing atleast one threshold level and an output signal only upon exceedance ofsaid at least one threshold level by said integrator output signal; andalarm means for providing an alarm indication in response to the outputsignal of said threshold means.
 11. An intrusion alarm system accordingto claim 10 wherein said threshold means is a bipolar threshold circuithaving a positive threshold for moving targets of one sense and anegative threshold for moving targets of opposite sense; and whereinsaid integrator means is operative to provide a unidirectional outputsignal of either positive or negative sense in accordance with the senseof target motion.
 12. An intrusion alarm system according to claim 10including: low pass filter means for receiving said rectangular shapedoutput signal from said limiting amplifier and for providing in responsethereto a bias signal for said pulse amplifiers to inhibit said samplepulses in the presence of spurious zero crossings of said rectangularshaped output signal caused by low Doppler frequency signals havingnoise components superimposed thereon.
 13. An intrusion alarm system fordetecting the presence of a moving target in the presence of interferingphenomena, said system comprising: means for transmitting a signal intoa zone under surveillance; means for receiving signals returned fromsaid surveillance zone; quadrature mixer means operative in response tosaid received signals and to a reference signal derived from saidtransmitted signals to provide first and second signals which includeDoppler signals in the presence of a moving target; means operative inresponse to said first signal to produce sample pulses which occur inregular timed relation in the presence of a moving target and at randomtimes in the presence of interfering phenomena, said sample pulseproducing means including: a limiting amplifier operative to provide arectangular shaped output signal in response to said first signal; adifferentiator circuit operative in response to said rectangular shapedoutput signal to provide pulses each occurring during a zero crossing ofsaid rectangular shaped output signal; pulse amplifier means operativeto amplify said differentiator pulses of at least one polarity toprovide said sample pulses; and low pass filter means for receiving saidrectangular shaped output signal from said limiting amplifier and forproviding in response thereto a control signal for said pulse amplifiermeans to inhibit said sample pulses in the presence of spurious zeroaxis crossings of said rectangular shaped output signal caused by lowDoppler frequency signals having noise components superimposed thereon;a limiter circuit operative in response to said second signal to providean output signal representation of said second signal limited to preventsignal excursions above predetermined reference levels; sample and holdmeans receiving said output signal representation from said limitercircuit and operative to store the representation of the amplitude ofsaid seconD signal at times specified by said sample pulses; anintegrator circuit for providing an integrated output signal in responseto said stored representation of the amplitude of said second signal,said integrated output signal being of unidirectional increasingmagnitude in the presence of a moving target and of randomly varyingmagnitude in the presence of interfering phenomena; and a thresholdcircuit having at least one threshold level which is outside the rangeof said randomly varying integrated output signal, and operative toproduce an alarm signal only upon exceedance of said at least onethreshold level by said integrated output signal.